Vertically integrated 3-axis MEMS angular accelerometer with integrated electronics

ABSTRACT

Sensors for measuring angular acceleration about three mutually orthogonal axes, X, Y, Z or about the combination of these axes are disclosed. The sensor comprises a sensor subassembly. The sensor subassembly further comprises a base which is substantially parallel to the X-Y sensing plane; a proof mass disposed in the X-Y sensing plane and constrained to rotate substantially about the X, and/or Y, and/or Z, by at least one linkage and is responsive to angular accelerations about the X, and/or Y, and/or Z directions. Finally, the sensor includes at least one electrode at the base plate or perpendicular to the base plate and at least one transducer for each sensing direction of the sensor subassembly responsive to the angular acceleration. Multi-axis detection is enabled by adjusting a configuration of flexures and electrodes.

FIELD OF THE INVENTION

The present invention relates generally to accelerometers and morespecifically to multi axis accelerometers that sense angular(rotational) accelerations.

BACKGROUND OF THE INVENTION

Angular or rotational accelerometers are used to measure rotationalacceleration about a specific axis. Rotational accelerometers have manyapplications such as vehicle rollover event prevention, rotationalvibration suppression for hard disk drives, airbag deployment and so on.With the advances in MEMS technology various rotational accelerometersthat can be fabricated using silicon micromachining techniques have beenproposed in U.S. Pat. No. 5,251,484, “Rotational accelerometer,” Oct.12, 1993; U.S. Pat. No. 6,718,826, “Balanced angular accelerometer,”Apr. 13, 2004; U.S. Pat. No. 5,872,313, Temperature-compensated surfacemicromachined angular rate sensor,” Feb. 16, 1999; U.S. Pat. No.6,257,062, “Angular accelerometer,” Jul. 10, 2001. In these applicationssurface micromachining used to fabricate the moving proof masses.Surface micromachining imposes limits on the structures. For example,the proof mass thickness is limited to the thickness of the depositedfilms. Surface micromachining also suffers for the stiction problem as aresult of sacrificial etching and wet release processes. Therefore proofmasses fabricated using this method requires additional supports aroundthe perimeter of the proof mass to reduce stiction and to increase thestability. This results in relatively more complicated devices andstringent requirements for the fabrication of additional springs thatwould not disturb the operation of the rotational accelerometer. On theother hand bulk micromachining overcomes most of the problems associatedwith the surface micromachining. U.S. Pat. No. 7,077,007, “Deep reactiveion etching process and microelectromechanical devices formed thereby,”Jul. 18, 1006 describes DRIE etching for bulk micromachined angularaccelerometers.

The sensing methods used in MEMS accelerometer vary. Capacitive sensorsprovide high performance as well as low cost. Because of these featuresit became the method of choice for most of the consumer marketapplications. But to be able to obtain high sensitivity and low noisefloor the parasitic capacitances need to be reduced or eliminated. Thiscan be achieved by integrating MEMS and electronics. The accelerometersdescribed in the above-identified patents are not integrated with thedetection electronics. In a typical system, the detection electronicsneeds to be connected to the MEMS substrate through wire bonding.Accordingly, this system suffers from increased parasitics and issusceptible to noise and coupling of unwanted signals.

Therefore, there is a need for rotational accelerometers that arefabricated using bulk micromachining methods and integrated withelectronics. There is also need for multi-axis accelerometers that areinsensitive to linear accelerations. The present invention addressessuch needs.

SUMMARY OF THE INVENTION

Sensors for measuring angular acceleration about three mutuallyorthogonal axes, X, Y, Z or about the combination of these axes aredisclosed. The angular accelerometers are fabricated by bulkmicromachining and integrated with electronics. The sensor comprises asensor subassembly. The sensor subassembly further comprises a basewhich is substantially parallel to the X-Y sensing plane; a proof massdisposed in the X-Y sensing plane and constrained to move substantiallyabout the X, and/or Y, and/or Z, by at least one linkage and isresponsive to angular accelerations about the X, and/or Y, and/or Zdirections. Finally, the sensor includes at least one electrode at thebase plate or perpendicular to the base plate and at least onetransducer for each sensing direction of the sensor subassemblyresponsive to the angular acceleration. Multi-axis detection is enabledby adjusting a configuration of flexures and electrodes.

Two structures or more can be used per axis to enable full bridgemeasurements to further reduce the susceptibility to power supplychanges, cross axis coupling and the complexity of the senseelectronics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows top view of a portion of a MEMS assembly according to anembodiment.

FIG. 1B shows the cross section AA′ of the angular accelerometer in FIG.1A.

FIG. 1C shows an angular accelerometer comprising two proof masses.

FIG. 1D shows the detection electronics for the accelerometer shown inFIG. 1C.

FIG. 2A illustrates an angular accelerometer composed of two proofmasses.

FIG. 2B shows an alternative arrangement of proof masses shown in FIG.2A.

FIG. 3A illustrates multi-axis accelerometer (X and Z rotationalaccelerometer, Z linear accelerometer).

FIG. 3B illustrates the cross section of the accelerometer shown in FIG.3A and the proof mass deflection as a function of input rotational X andlinear Z acceleration.

FIG. 3C illustrates one proof mass that is sensitive to rotational X andZ accelerations but insensitive to linear accelerations.

FIG. 4A illustrates four axis accelerometer (X, Y and Z rotational, Zlinear accelerometer).

FIG. 4B illustrates an alternative arrangement of proof masses shown inFIG. 4A.

FIG. 4C shows the detection electronics for the Z-axis angularaccelerometers shown in FIGS. 4A and 4B.

FIG. 5 illustrates one variation of flexures.

DETAILED DESCRIPTION

The present invention relates generally to motion sensing devices andmore specifically to angular accelerometers utilized in integratedcircuits. The following description is presented to enable one ofordinary skill in the art to make and use the invention and is providedin the context of a patent application and its requirements. Variousmodifications to the preferred embodiments and the generic principlesand features described herein will be readily apparent to those skilledin the art. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features described herein.

A method and system in accordance with the present invention relates tothe accelerometers that are fabricated using silicon micromachiningmethods that have been described in U.S. Pat. No. 7,104,129, entitled“Vertically Integrated MEMS Structure with Electronics in a HermeticallySealed Cavity,”, issued Sep. 12, 2006, and assigned to the assignee ofthe present application; and U.S. Pat. No. 7,247,246, entitled “VerticalIntegration of a MEMS Structure with Electronics in a HermeticallySealed Cavity,” issued Jul. 24, 2007, and assigned to the assignee ofthe present application, both of which are incorporated by reference intheir entirety herein. The assembly approach (Nasiri fabricationprocess) described in the said patents provides a cost effective meansto simultaneously protect the movable sensing element and to integratethe low noise electronics. The electronic circuitry is fabricated on adedicated electronics silicon substrate. The MEMS assembly then bondedon the electronic or sense substrate using a metal bonding techniqueusing a low temperature process that does not damage or compromise theelectronic circuitry. A plurality of transducers is assembled in thismanner at the wafer level where hundreds to thousands are producedsimultaneously. A small size form factor is achieved by the verticalintegration of the sensing element with its sensing circuit. Otherpatents that are relevant for accelerometer fabrication are: U.S. Pat.No. 6,939,473 “Method of making an X-Y axis dual-mass tuning forkgyroscope with vertically integrated electronics and wafer-scalehermetic packaging”; U.S. Pat. No. 7,258,011 “Multiple axisaccelerometer”; and U.S. Pat. No. 7,250,353 “Method and system ofreleasing a MEMS structure” assigned to the assignee of the presentapplication.

FIGS. 1A and 1B show a rotational accelerometer 100 and the crosssection AA of the accelerator 100, respectively. As is seen, a proofmass 102 is attached to the cover plate 104 at a single anchor 106through flexural springs 108. The anchor 106 is at the center of theproof mass 102. The proof mass 102 is movable. It is constraint torotate along Z axis which is perpendicular to the proof mass 102. Thesprings 108 can be made in any shape to adjust the spring constant. Theanchor 106 is attached to the cover plate 104 utilizing a bondingprocess such as fusion bonding. The anchor 106 is also connected to thesense substrate 118 through an electrical connection 112.

The electrical connection 112 can be made under the anchor 106 asdescribed in published U.S. Published Application No. 2006/0208326,entitled “Method of fabrication of al/ge bonding in a wafer packagingenvironment and a product produced therefrom” which is also assigned tothe assignee of the present application. The method described in thatpublished patent application allows making mechanical and electricalconnections on the same anchor. The single anchoring of the proof mass102 reduces the stresses that may be induced by the package warpage. Thesense electrodes 114 a and 114 b are coupled to sensor substrate 118 anddo not move with respect to proof mass. When the proof mass 102 issubjected to an angular acceleration about the axis (Z-axis)perpendicular to the plane, the forces acting on the proof mass 102rotates it about the anchor 106. The rotation of the proof mass 102 issensed capacitively. The moving electrodes 116 extending from the proofmass 102 form capacitors with the sense electrodes 114.

The sense electrodes 114 a-114 b are bonded to the cover plate 104 andelectrically connected to the sense substrate 118. There are two senseelectrodes 114 a-114 b per each moving electrode 116 on the proof mass102 forming two capacitors. The value of one of these capacitorsincreases, whereas the value of other one decreases as the proof mass102 rotates. The capacitors are labeled C_(CW) 120 and C_(CCW) 122 asshown in FIG. 1A.

C_(CW) 120 increases if the proof mass 102 rotates in clock wisedirection and C_(CCW) 122 increases if the proof mass 102 rotates in thecounter-clockwise direction about the axis perpendicular to the sensesubstrate. C_(CW) 120 and C_(CCW) 122 allow for the differentialdetection of the proof mass 102 rotation and hence provide an indicationof the angular acceleration.

Referring to FIG. 1A, the accelerometer 100 has features to providereliable operation. For example, it has motion stoppers 124 a-124 babout Z rotation to restrict the motion for excessive acceleration. Inthe directions that are out of plane, its stiffness is high enough toprovide mechanical stability. The accelerometer 100 has also self testelectrodes 126 to actuate the proof mass 102 for test purposes.

As before mentioned, FIG. 1B shows the cross section of the angularaccelerometer shown in FIG. 1A. Other MEMS devices such asaccelerometers and gyros have been disclosed previously by the assigneeof the present application (U.S. Pat. No. 7,258,011, “Multiple axisaccelerometer”; U.S. Pat. No. 6,939,473, “Method of making an X-Y axisdual-mass tuning fork gyroscope with vertically integrated electronicsand wafer-scale hermetic packaging”; U.S. Pat. No. 6,892,575, “X-Y axisdual-mass tuning fork gyroscope with vertically integrated electronicsand wafer-scale hermetic packaging”). A similar fabrication platformdescribed in these patents may also be used for the angularaccelerometers shown in FIG. 1B.

The fabrication process starts with the manufacturing of the cover plate104. First alignment marks are patterned on top of the cap or coverwafer. These marks will be later used to align the cover wafer to thesense substrate. Then the cover plate 104 is oxidized preferably usingthermal oxidation to form an oxide layer. The preferable thickness ofthe oxide is between 0.5 and 1 micron. The oxide is patterned usinglithographic methods to define the cavities in the cover plate 104.

The cavity depth can be further increased by etching the exposed siliconsurfaces in the cover plate 104. But, if the structures in the actuatorlayer are not supposed to move more than the oxide thickness in verticaldirection or there are no difficulties associated with having a cover inthe close proximity of the moving parts, the silicon etch step may beskipped.

Then, the cover plate 104 is cleaned and bonded to another low totalthickness variation wafer. The second wafer will form an actuator layerafter thinning it down to preferably 40 microns. The actuator layerincludes the proof mass 102 the sense electrodes 114 a and 114 b and theflexure springs 108 any other structures such as self test electrodesand over travel stoppers. The next step in the process is the formationof the stand offs. An etch, such as an KOH etch, is suitable for thisstep. The height of the stand offs determine the vertical separationbetween actuator layer and the sense substrate 118. If there areelectrodes on the sense substrate 118, this gap also determines thesensing capacitor gaps. Then, a germanium (Ge) layer is deposited andpatterned. In the next step, elements of the rotational accelerometerare defined lithographically and etched using DRIE in the actuatorlayer. In the final step, the actuator layer is bonded a sense substrateusing eutectic bonding.

Accordingly, as is seen in FIG. 1B, the active areas of the sensesubstrate 118 include regions that will make electrical contact with anactuator layer where the angular accelerometer 100 is defined, as wellas circuitry 125 for sensing output signals from the angularaccelerometer 100. Such circuitry 125 is preferably conventional CMOScircuitry. The top layer 127 of metal deposited in the conventional CMOSprocess is suitable for use as a bond metal. This upper layer 127 ofmetal defines bond pads for the connections to the sense electrodes 114a and 114 b and the proof masses 102. One can also put electrodes on thetop layer 127 to measure out of plane motion of the accelerometer in thecase X and Y angular accelerometer.

The connections to the proof masses 102 and sense electrodes 114 a and114 b can be routed in the lower CMOS metals 129, 131 and 133 wheremetals can cross over each other in different layers. This allows forcomplicated routing schemes to be utilized for connecting the MEMSdevice to the active electronics. Another advantage of having the sensesubstrate 118 in the close proximity of the angular accelerometer isthat the connections between the MEMS device and sense electronics canbe made very short. This reduces the parasitic coupling to ground, crosscoupling between the wires and EMI coupling.

The above-described fabrication process produces hermetically sealedsensors for example utilizing sealing rings 135. The sense substrate 118is preferably attached to the actuator layer via a metal-to-metal bond,which can be made hermetic. Likewise, the actuator layer is preferablyattached to cover plate 104 by a fusion bond, which can also be madehermetic. As a result, the entire assembly of sense substrate 118,actuator layer and cover plate 104 can provide a hermetic barrierbetween angular accelerometer elements and an ambient environment. Thepressure in the cavity can be adjusted during the eutectic bondingprocess. This allows the quality factor of the angular accelerometer tobe controlled for better noise performance and dynamic response.

The above-described fabrication process also allows combining variousinertial measurement devices on the same substrate. The angularaccelerometers described in this patent can be easily integrated withlinear accelerometers as well as low cost gyroscopes.

One can use two of the structures shown in FIG. 1A to provide fourchanging capacitances as shown in FIG. 1C. Note that, in this case theproof masses are electrically isolated. This allows for the detection ofa capacitance change utilizing a full bridge configuration in so doingcommon mode signals are eliminated and allows using simpler electronicscan be utilized. The capacitance change of the accelerometers describedabove can be detected by various circuits.

An example of circuitry for detecting the capacitance change due torotational acceleration is shown in FIG. 1D where a full bridgeconfiguration is utilized. As is seen, AC voltages 201 a, 201 b, whichare 180 degree out of phase with respect to each other are applied tothe proof masses 202 a and 202 b. The output voltage is detected off thesense electrodes utilizing an operational amplifier 204. When there isno acceleration, the bridge is in balanced and the output voltage iszero. Angular acceleration of the proof masses 202 a and 202 b disturbsthe balance and gives rise to an AC voltage at the operational amplifier204 output which amplitude is proportional to the acceleration. Theoperational amplifier 204 output later can be demodulated to obtain asignal directly proportional to the acceleration.

In this embodiment, a full bridge circuit is described but one ofordinary skill in the art readily recognizes other means of capacitivedetection such as pseudo bridge, half bridge can also be employed.Alternatively, one can also drive the sense electrodes and monitor theproof mass motion and by observing the output voltages of the op-amp.

In an alternative configuration to obtain full bridge configuration,instead of using the full circular proof mass of FIG. 1A, one can useonly half of the proof mass 302-304. This reduces the area usage asshown in FIG. 2A at the expense of sensitivity. Each proof mass 302-304is connected to a single anchor point by three or more flexures. In thiscase C_(CW1) 306 and C_(CW2) 308 increase with the clock wise rotationof the proof mass 302 and 304 whereas C_(CCW1) 310 and C_(CCW2) 312decrease. The change in the capacitance can be detected in a full bridgeconfiguration as shown in FIG. 1D. FIG. 2B shows an alternativeplacement for the proof masses of FIG. 2A.

In another configuration as shown in FIG. 3A, the flexures can beconfigured such that the proof masses 402′ and 404′ become sensitive torotation about another axis (X) in addition to the first rotational axiswhich is Z in this case. The Z-rotation detection method is same as thescheme described in FIG. 2. However, attaching the proof masses 402 and404 with flexures along the edge of the half circle makes them sensitiveto the rotations about the axis parallel to that edge. For Z axisrotation, the flexures simply flex allowing the rotation of the proofmass 402 and 404. For X axis rotation, the flexures make a torsionalmotion. The out of plane motion of the proof masses 402 and 404 can bemeasured by parallel plate capacitors between the proof masses 402 and404 and the sense substrate.

In FIG. 3A, the capacitance between the proof mass 402 and the substrateis C_(PM1) 414 and the other capacitor is C_(PM2) 416 which is betweenthe proof mass 404 and sense substrate. The rotational accelerationabout X moves one of the proof masses away from the substrate and theother one closer to the substrate. This increases the capacitanceC_(PM2) 416′ and reduces the C_(PM1) 414′ according to FIG. 3B. Thesetwo capacitors can be used for differential detection of the rotation.Again, by replicating the structure shown in FIG. 3A, one can obtainfour capacitances for X axis rotation to implement full bridgedetection. However, for Z rotation, one structure as shown in FIG. 3A isenough to implement full bridge configuration, but two structureconfiguration also improves Z sensitivity. In addition to rotationalaccelerations, these accelerometers can be used to measure linearacceleration along Z direction as shown in FIG. 3B. In this case, thesum of the C_(PM1) 414′ and C_(PM2) 416′ needs to be detected, ratherthan the difference of them which is the case for measuring rotationalacceleration about the X axis.

Alternatively, one can use the structure shown in FIG. 3C for X and Zrotation where X direction is in plane and parallel to the flexure 612and Z direction is perpendicular to the lateral plane. In thisstructure, the two proof masses (402, 404) of FIG. 3A are combined toform a single proof mass 610. The proof mass 610 is constraint to rotateabout the anchor 618 and about the flexure 612. Electrodes 602 and 604are sensitive to rotations about Z. The electrodes 606 and 608 which arebetween the sense substrate and the proof mass 610 are sensitive torotations about X. However, for this structure linear acceleration alongZ direction will not result in any capacitance change on C_(PMP) andC_(PMN) therefore this accelerometer is insensitive to linearacceleration along Z. Full bridge configuration will require two ofthese structures.

FIG. 4A shows a three axis rotational accelerometer. There are fourproof masses 702, 704, 706 and 708. The Z rotation is detected throughC_(CW1) 710, C_(CCW1) 712, C_(CW2) 722, C_(CCW2) 724, C_(CW3) 726,C_(CCW3) 728, C_(CW4) 730, and C_(CCW4) 732 capacitors. One can easilyconstruct full bridge configuration for this case as shown in FIG. 4C.Basically, proof mass 702 and 706 are connected in parallel likewiseproof mass 704 and 708. The rotations about X and Y are sensed throughcapacitors C_(PM2) 716, C_(PM4) 720 and, C_(PM1) 714, C_(PM3) 718respectively. When there is rotation about positive X direction, C_(PM2)716 increases and C_(PM4) 720 decreases. Since the rotation axis isthrough the centers of C_(PM1) 714 and C_(PM3) 718 these capacitors donot change. Similarly, for Y axis rotation only C_(PM1) 714 and C_(PM3)718 change, but C_(PM2) 716 and C_(PM4) 720 remain the same. Theaccelerometer shown in FIG. 4A also sensitive to Z axis linearaccelerations. This acceleration can be detected by sensing the sum ofC_(PM1) 714, C_(PM2) 716, C_(PM3) 718 and C_(PM4) 720. In an alternativeconfiguration shown in FIG. 4C, the orientation of the proof masses arechanged. Placing the X and Y axis rotation detection sensors away fromthe center of the device increases the sensitivity.

FIG. 5 shows an example of a proof mass 800 which includes a differenttype of flexure 802. One can use folded springs to tailor the springconstant. A folded spring can be connected to the proof mass at thecenter as shown in FIG. 5. This configuration allows obtaining smallspring constants for increased sensitivity in small areas.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A rotational accelerometer comprising: a cover substrate, a sensesubstrate; and an actuator layer; the actuator layer comprising at leastone proof mass constrained to rotate about at least one axis the atleast one proof mass being anchored to the cover and to the sensesubstrate via at least one flexure, and a plurality of electrodes thatcan sense rotation about the said axis capacitively.
 2. Theaccelerometer of claim 1 wherein the cover substrate and the sensesubstrate provide for a hermetically sealed controlled enclosure for theaccelerometer.
 3. The accelerometer of claim 1 wherein the at least oneproof mass comprises at least two proof masses.
 4. The accelerometer ofclaim 1 wherein the at least one proof mass is constrained to rotatesubstantially about Z-direction to measure rotational acceleration aboutthe Z-direction, and is constrained to rotate about substantially forX-direction to measure rotational acceleration about the X-direction. 5.The accelerometer of claim 1 wherein the at least one proof mass isconstrained to rotate substantially about Z-direction to measurerotational acceleration about the Z-direction, and is constrained torotate about substantially for Y-direction to measure rotationalacceleration about the Y-direction.
 6. The accelerometer of claim 3wherein the at least one of the two proof masses is constrained torotate about substantially Z-direction to measure rotationalacceleration about the Z-direction, and is constrained to rotate aboutsubstantially for a X-direction to measure rotational acceleration aboutthe X-direction and the other of the two proof masses is constrained torotate about substantially Z-direction to measure rotationalacceleration about the Z-direction, and is constrained to rotate aboutsubstantially for Y-direction to measure rotational acceleration aboutthe Y-direction.
 7. The accelerometer of claim 1 wherein the at leastone electrode forms a parallel plate capacitance.
 8. The accelerometerof claim 3 wherein a full bridge configuration is utilized forcapacitive measurement of the at least two proof masses.
 9. Theaccelerometer of claim 3, wherein a full bridge configuration isutilized for capacitance measurement of the at least two proof massesconstrained to rotate about the Z-direction.
 10. The accelerometer ofclaim 3, wherein a full bridge configuration is utilized for capacitancemeasurement of the at least two proof masses constrained to rotate aboutthe Y-direction.
 11. The accelerometer of claim 3, wherein a full bridgeconfiguration is utilized for capacitance measurement of the at leasttwo proof masses constrained to rotate about the X-direction.
 12. Theaccelerometer of claim 1 which includes a self-test electrode to actuatethe at least one proof mass for test purposes.
 13. The accelerometer ofclaim 1 which includes travel stoppers in at least one of an X or a Y orZ direction.
 14. The accelerometer of claim 1 wherein the at least oneproof mass is connected electrically to the sense substrate at ananchor.
 15. The accelerometer of claim 1 wherein the at least one proofmass is connected electrically to the sense substrate through the atleast one flexure.
 16. The accelerometer of claim 1 wherein the sensesubstrate comprises a single crystal silicon wafer.
 17. Theaccelerometer of claim 1 wherein the sense substrate comprises a CMOSintegrated circuit.
 18. The accelerometer of claim 17 wherein ameasurement is provided utilizing a half bridge configuration with thecapacitance being in series with a constant capacitance.
 19. Theaccelerometer of claim 17 wherein a measurement is provided utilizing afull bridge configuration with the capacitance being with three othercapacitances.
 20. The accelerometer of claim 18 wherein the capacitancemeasurement for the half bridge configuration is proof mass driven. 21.The accelerometer of claim 18 wherein the capacitance measurement forthe half bridge configuration is electrode driven.
 22. The accelerometerof claim 19 wherein the capacitance measurement for the full bridgeconfiguration is proof mass driven.
 23. The accelerometer of claim 19wherein the capacitance measurement for the full bridge configuration iselectrode driven.